Future information networks require two modes of operation, these are connectionless datagram and connection-oriented virtual circuit modes. Both these modes are well-known. The best known example of a connectionless datagram architecture is the internet protocol (IP), while the corresponding example of a connection-oriented virtual circuit architecture is asynchronous transfer mode (ATM) technology.
Connectionless IP is far more efficient than a virtual circuit architecture such as ATM for browsing, e-mail applications, and other non-realtime services, but it is limited in capacity by its router-based architecture. Fixed path virtual circuit architectures such as ATM are suitable for services which demand quality guarantee and real-time delivery, but they are inefficient for many non-realtime services due to their path set-up requirements.
However given the great popularity of the Internet and given the widespread acceptance of ATM as the broadband standard, both modes are likely to stay. Attempts to replace one with the other have failed.
For example, consider the resource reservation protocol (RSVP) proposed for the Internet. The idea of RSVP was to add resource reservation to IP so as to support real-time services such as audio or video links. The problem is that IP's routers change paths constantly, which is in the very nature of a connectionless network, and although it was initially argued that reservation could be achieved without fixing the path of a connection, it was subsequently realised that this may not be possible. Fixing the path has been proposed, but this creates more problems than it solves with the Internet losing the flexibility of a connectionless network. Even with path fixing the network still needs admission control, rate policing and so on to guarantee quality of service.
Similarly various proposals have been made to support connectionless service with ATM. These proposals include LAN emulation or IP-over-ATM, but in all cases the solutions do not escape the need to set up paths for connectionless services. They are highly inefficient and fail to break the capacity bottleneck of the current Internet. In the event of congestion, a single cell loss leads to loss of the entire IP packet (which can be as long as 1200 cells). When one cell is dropped transmitting the remaining cells achieves nothing but bandwidth waste. Additional measures, such as early packet discarding, must be taken to tame the problem. These proposals all add additional states to each virtual channel and additional complexity to the system.
Individually ATM and IP have their own drawbacks. In an ATM network to prevent bandwidth waste during silent periods statistical multiplexing for real-time traffic is required. However statistical multiplexing also demands complex methods for policing, billing, and admission control. In addition setting up a virtual circuit is a very wasteful way to support browsing and transaction type non-realtime services.
However, on the other hand, as the popularity of the Internet soars conventional IP-router implementation has reached its capacity limit and any significant increase in capacity is unlikely with current IP architectures. Furthermore bandwidth reservation is required for real-time services, but this is difficult to implement in a connectionless environment where the path changes constantly.